I feed a fairly low protein pellet. Both new horses are very underweight, so cutting that out isn't an option. (Yet.) We don't feed oats/sweet feed down here. Its a local companies feed, mainly its a pelleted combination, but different in a way. Sorry lol. He's on a daily probiotic and skin/hair supplement.
Well, we can rule out "grain brain". And probiotics are great. Does the skin/hair suppliment have omega fats? Sometimes imbalance in fats can make for hormone swings. I actually like Chia seeds for that, and it helps keep the digestion moving along, but you can do flaxseed, too, but I think the chia seeds are more 'bang for your buck' in terms of nutritional content per serving. A small amount of seeds goes a long way, especially for those moody lady horses (and lady people!).
Well, I like the Quietex paste for a potentially stressful situation, but I only use it for trailering, not on a daily basis. I know there are a lot of other brands out there, and we would probably shop around except for the fact we know the Quietex works for the trailer ride. Go with what works, right? You have to remember that the product needs time to work, like 1 or 2 hours, so you have to be mindful and use it before the stressful situation, like vet or farrier visits or trailering.
HOWEVER, if you are looking for an every day product, I think Quiessence would be the best bet. It is not for just nervousness because Chromium and Magnesium help with all cellular processes - they're not only needed for the growth and maintenance of healthy bones and teeth but are also essential in the synthesis of nucleic acid and proteins. Chromium and Magnesium are great for a lot of nerve issues and also help with muscle repair. Magnesium also aids in digestion and helps keep the colon moving (great for people, too). Plus, these aren't 'drugs'. They are necessary minerals for the nervous system, cells, and heart. Magnesium is great for relaxing muscles (If you ever have very sore muscles, take some Magnesium powder in water before bed and you will feel better in the morning, and it is SO MUCH BETTER for your LIVER than Advil!). But they should take the 'edge' off when given on a daily basis as a suppliment.